Emily, go to Host, Calender, Availability Settings, scroll on the menu to the bottom and you will see Sync Calender. Import or Export from/to the other listing.
However in your case, since Entire is the less frequently used options. Block it for all the dates that the Guest can book in advance (normally default to 6 months or as decided when you set up your listing). when you are out of town or travel, then unblock those dates and go to the private Room listing to block it. You will have to do it manually and goes to BOTH listing to update the Calender.
You only select Sync Calender if you are always having both listing available for guest to book.